This invention relates generally to thin film phase change memories.
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, as an electronic memory. One type of memory element utilizes a phase change material that may be, in one application, electrically switched between generally amorphous and generally crystalline local orders or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states.
Typical materials suitable for such an application include various chalcogenide elements. The state of the phase change materials is also non-volatile. When the memory is set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance or threshold voltage (Vt) value, that value is retained until reprogrammed, even if power is removed. This is because the programmed value represents a phase or physical state of the material (e.g., crystalline or amorphous).
Initially, in the manufacture of thin film phase change memories, all the devices are fabricated in the low resistance or set state. Generally, an access device or select transistor is utilized in series with the memory element to isolate the memory element. The select device provides isolation (very high resistance when not selected). Even if all the the phase change memory cell memory elements, as fabricated, come out in the fully crystalline or low resistance state there will be no problem initially programming large arrays from the very low multiple resistance paths between all of the word lines and bit lines because the necessary resistance can be provided by the select devices. The presence of the select devices may increase the number of processing steps and may increase the size of the cell and, therefore, the size and cost of the overall memory.
Thus, there is a need for a way to make thin film phase change memory devices without an access or select device.